High performance fingerprint imaging system

ABSTRACT

A system for optically imaging an object includes an optical platen having an object receiving surface. The object receiving surface is illuminated by a multi-color light source, and a color imaging system forms an image of the object on the object receiving surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 119(e) to theprovisional application entitled: “HIGH PERFORMANCE FINGERPRINT IMAGINGDEVICE UTILIZING COLOR IMAGERS, COLOR CORRECTED OPTICS, AND WHITE LEDLIGHT SOURCE”, U.S. Ser. No. 60/624,644, filed Nov. 2, 2004, which isincorporated herein by reference.

TECHNICAL FIELD

This invention relates to imaging devices for use, for example, withfingerprint imaging systems.

BACKGROUND

Up-to-date fingerprint imaging systems using fingerprint image transferinto electronic data usually apply the known contact method to create afingerprint pattern. A surface topography of a finger is approximated bya series of ridges with intermediate valleys. When a finger is appliedto a surface of a transparent optical plate or prism, the ridges contactthe optical plate while the valleys do not and instead serve to form theboundaries of regions of air and/or moisture.

The finger to be imaged is illuminated by a light source located belowor near the optical plate. Imaging light from the light source isincident on the surface of the optical plate at an angle of incidencemeasured with respect to a normal to that surface. Imaging lightreflected from the surface is detected by an imaging system thatincludes some form of a detector.

Components of a typical fingerprint imaging system are oriented so thatan angle of observation (defined to be an angle between an optical axisof the imaging system and the normal to the optical plate surface) isgreater than a critical angle for the interface between the surface andair at the surface. The critical angle at the surface/air interface isdefined as the smallest angle of incidence for which imaging lightstriking the surface/air interface is totally internally reflected (TIR)within the optical plate. Therefore, the critical angle at thesurface/air interface depends on the index of refraction of the air andthe optical plate. Another constraint for the angle of observationarises because there is incentive to observe the image at the smallestpractical angle of observation, as this reduces distortion due to objecttilting. Therefore, the angle of observation is typically chosen to beclose to, but greater than the critical angle at the surface/airinterface.

A livescan imaging system may be configured to capture four-finger slap,single-finger slap, and rolled fingerprint images. Conventionalfingerprint imaging systems of this type may provide 500 pixels per inch(ppi) images. However, it is also desirable to provide more detailedimages such as 1,000 ppi images.

Conventional fingerprint imaging platforms use monochrome chargedcoupled device (CCD) imagers, monochromatic light sources, andanamorphic correcting optics to map an object plane to an image plane.CCD images and electronics, however, are expensive. Optics andopto-mechanics are also expensive, and a monochromatic light sourceproduces light of only one color.

Another platform uses dual, small-format, low frame rate (about 4-5frames per second (fps)), 1.3 megapixel complementary metal-oxidesemiconductor (CMOS) color imagers, a monochromatic light source andmonochromatic optics. The object plane is split with each half mapped toone of the pair of imagers. However, performance is not improvedrelative to other, conventional designs. Low frame rates also produceartifacts and the use of a monochromatic light source limitssignal-to-noise in blue and red pixels. Additionally, relatively lowimager pixel count limits the contrast transfer function (CTF).

Other livescan systems use two separate imaging chains, one for rolledimages and one for four-finger slaps. CCD imagers are employed.Illumination is monochromatic. The four-finger slap imager employs aCMYG (cyan-magenta-yellow-green) color matrix CCD. The four-finger slapimager uses a red monochromatic light source which strongly stimulatesthe magenta and yellow pixels but weakly (if at all) the green and cyanpixels. Such weak pixel performance must be compensated for by strongequalization producing a pixel dependent noise pattern, or byinterpolating strong pixel values to create or enhance weak pixelvalues. This technique results in inferior performance. Also, thefour-finger slap imager provides a low frame rate (about 4 fps),yielding motion artifacts when the fingers move while forming the image.

Still other systems use a light pipe illumination scheme. However, thesurface to be illuminated is small (1.6×1.5 inches) and the system ismonochrome using light having a wavelength of about 650 nanometers (nm).One such system employs object plane telecentric optics. Another uses apair of cylinder lenses to provide anamorphic distortion to map theobject plane format to the image plane format using the maximum numberof pixels (non-square pixels). Another system makes use of a prism pairto anamorphically distort the image in the vertical domain to map theobject plane exactly to the image plane to accomplish exactly 500 ppi,square pixels.

SUMMARY

In one aspect, the invention features a system for optically imaging anobject. The system includes an optical platen having an object receivingsurface. A multi-color light source is positioned to illuminate theobject receiving surface. A color imaging system having an image planeis positioned to receive light from the object receiving surface to forman image of the object on the object receiving surface. A lens mechanismis provided to focus light from the object receiving surface onto theimage plane and to provide color correction of the focused light.

Various implementations of the invention may include one or more of thefollowing features. The lens mechanism includes an achromat. The lensmechanism includes either one pair of doublets or one pair of doubletsand a pair of singlets. The lens mechanism removes a substantial portionof chromatic aberration. The lens mechanism includes an aperture. Thesystem further includes a folding mirror to direct light from the objectreceiving surface to the lens mechanism. The system may incorporate twofolding mirrors. The light provided by the light source has a wavelengthof between about 450 and 650 nanometers. The system includes a pair ofCMOS imagers. The imagers are tilted at an angle from a normal. Thesystem includes either a CCD or CMOS imager. The system produces 500 and1,000 pixels per inch images. The system is configured to capture atleast four-finger slap, single-finger slap, and rolled fingerprintimages. The light source is a white; red and green; blue and green;cyan, magenta and green; cyan, green and yellow; or green, yellow andmagenta light source. The light source is selected from a groupconsisting of a light emitting diode, a cold cathode fluorescent tube,or a plasma panel illuminator. The object is a finger.

In yet another aspect, the invention features a system for opticallyimaging features on a surface of a hand. The system includes an opticalplate means for forming a finger receiving surface. A non-monochromaticlight source means is used to illuminate the finger receiving surface. Acolor imaging means receives light from the finger receiving surface toform an image of a finger on the finger receiving surface. A lens meansfocuses light from the object receiving surface onto an image plane ofthe color imaging means and provides color correction of the focusedlight.

In still another aspect, the invention features a method of imaging anobject. The method comprises receiving an object at an object receivingsurface of an optical platen. The object receiving surface isilluminated with a multi-color light source. Light from the objectreceiving surface is collected. The collected light is color correctedand focused onto an image plane of a color imaging system to form animage of the object.

Various implementations of the invention may include one or more of thefollowing features. The received object is a finger. Light from thelight source illuminating the optical platen is incident on the opticalplaten at an angle with respect to a normal to the object receivingsurface which is less than a particular critical angle.

In a further aspect, the invention features an illumination source. Theillumination source comprises a light output surface and a lightreceiving surface located substantially orthogonal to the light outputsurface. A diffusing structure is at the light output surface. Anon-monochromatic light source is located adjacent to the light outputsurface.

Various implementations of the invention may include one or more of thefollowing features. The diffusing structure is an array of microprisms.The light source produces white light; red and green light; blue andgreen light; cyan, magenta and green light; cyan, green and yellowlight; or green, yellow and magenta light. A light source is positionedin respective reflecting end caps located at opposite sides of a cavityformed between the light output surface and a back surface. The backsurface is opaque, and the light output surface is clear.

The invention can include one or more of the following advantages. Thesystem supports 500 ppi and 1,000 ppi image capture for four-fingerslap, single-finger slap, and rolled finger images with frame rates highenough to avoid artifacts. An increased CTF, up to three times greaterthan conventional 500 ppi imaging systems, is provided in the 500 ppimode. The system, in the 1,000 ppi mode, meets or exceeds FBI ElectronicFingerprint Transmission Specification (CJIS-RS-0010) Appendix F at allstimulus frequencies up to and including the frequency where samplingengenders aliasing. The system achieves geometric accuracy better thanAppendix F requirements for the 1,000 ppi mode. The system also achievessignal-to-noise performance that is better than Appendix F requirements.The system performs as if it were monochrome in construct. However, thecost of the optics, opto-mechanics, and imager electronics are reducedto levels comparable to or less than conventional 500 ppi fingerprintimaging systems.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows schematically a side sectional view of an imaging systemaccording to the present invention.

FIG. 2 schematically illustrates a perspective view (without theillumination source) of the imaging system of FIG. 1.

FIG. 3 schematically illustrates the illumination source of the imagingsystem of FIG. 1.

FIG. 4 schematically illustrates a processing system for the imagingsystem of FIG. 1.

DETAILED DESCRIPTION

As there are advantages to using the fingerprint as an identifier, whichcannot be forgotten or lost, the field of application for fingerprintimaging devices is constantly expanding. For example, a fingerprint maybe used as an access key.

A fingerprint imaging device may be used to capture four-finger slap,single-finger slap, and rolled fingerprint images. A fingerprint imagingdevice may also be used to capture palm and full-hand images. Ideally,such a device should not only produce 500 ppi images but 1,000 ppiimages as well. Both images should meet or exceed the specifiedrequirements or standards, for example, the FBI Electronic FingerprintTransmission Specification (CJIS-RS-0010) Appendix F requirements, atall stimulus frequencies up to and including the frequency wheresampling engenders aliasing. The fingerprint imaging device should alsosupport higher resolution images and be compatabile with evolvingstandards.

As shown in FIGS. 1 and 2, such a fingerprint imaging apparatus orsystem 100 includes an optical plate or platen 102, a primary lens 104,a mirror system 106, an objective lens system 107, an image sensorsystem 108, and an illumination source 110. For further reference,directions Y and Z of the orthogonal coordinate system are shown on FIG.1 by arrows. A third direction X of this orthogonal coordinate system isperpendicular to the drawing plane of FIG. 1.

The optical platen 102 includes an object receiving surface or fingerfield 112 located on its top. An object, such as a finger, to beidentified is applied to the surface 112. The surface 112 comprises theobject plane of the system 100.

The finger field 112 has an optically smooth surface to provide goodcontact with the finger skin ridges. The finger field regions thatinterface with the finger skin ridges and valleys form the fingerprintpattern. The finger field has dimensions sufficient for reliableidentification of the fingerprint pattern. The object receiving surfaceis large enough to provide sufficient X-Y dimensions to image fourfingers at the same time as well as rolled fingerprint images. Thesurface of the object receiving surface in the X-Y plane may, forexample, be about 86 millimeters (mm) in length (the X-direction) andabout 66 mm in width (the Y-direction).

The optical plate or prism 102 includes a first side surface 114 and asecond side surface 116 inclined to the finger field 112. The fingerfield and the surfaces 114 and 116 are planar in shape. Other shapes arepossible for either or both of these surfaces, such as, for example,cylindrical shapes, to enhance various characteristics of thefingerprint image.

The surface 114 is configured to receive a source of illumination forthe object plane surface 112. The surface 116 is the viewing facethrough which the illuminated object plane is viewed by TIR. The surface114 is inclined to the finger field at an angle 118, as shown in FIG. 1.The value of the angle 118 is generally chosen to accomplish the desiredillumination of the object plane. The surface 116 is inclined to thefinger field at an angle 120. The object plane surface 112 isilluminated through the illumination face 114 of the prism at an anglein the range of 25 to 40 degrees(°). The angles 118 and 120, in oneembodiment, may be approximately 40 and 50°, respectively. Thisembodiment represents bright field illumination. Dark field illuminationwhereby the object plane is illuminated at an angle approximating 90°may also be utilized. In this case, the finger friction ridges are seenas a bright object on the dark field background. The dark fieldimplementation may be preferable in some cases where packaging of theoptics dictates that the illumination be provided from other than afront surface of the object plane prism or plate.

The primary lens 104 is positioned external to the optical plate 102 andbehind its lateral surface 116. The primary lens 104 may comprise, forexample, a square field lens. The field lens accomplishes telecentricityof rays at the object plane. The lens directs light from the objectplane to the objective lens system 107.

When a finger is applied to the object plane, finger ridge detail isviewed by frustrated total internal reflection (FTIR). The optical plateor prism in one embodiment employs the principle of moisturediscrimination whereby the index of the refraction of the glass and theviewing angle of the object plane can discriminate the index ofrefraction of skin from that of both air and water at the object planesurface. This technique is described in U.S. Pat. No. 5,416,573,entitled “Apparatus For Producing Fingerprint Images Which AreSubstantially Free Of Artifacts Attributable To Moisture On The FingerBeing Imaged”, assigned to the assignee of the subject application, andwhich is herein incorporated by reference.

Specifically, in one embodiment, the image sensor system 108 receiveslight from the platen surface where air or water is in contact with thatsurface, but receives significantly less light from regions of theplaten surface where friction ridge skin is in contact. Generally,moisture discrimination is implemented with high index of refractionglass to implement TIR with acceptable geometric distortion. Lower indexglass is also feasible with associated techniques to correct forgeometric distortion and provide acceptable contrast transfer function(CTF) at the associated steeper viewing angle.

The prism may comprise SF-11 glass (index of refraction=1.785) readilyavailable from high quality glass fabricators. The viewing angle 120 is,in one embodiment, as noted, is approximately 50° to accomplish moisturediscrimination. A lower index glass may be used, for example, BK7 glass,and the object plane viewed at a lower angle, for example, at an angleof approximately 65°, to accomplish FTIR for skin and water applied tothe object plane. Other transparent materials, glass or plastics, forinstance, may be used in place of these specific glasses.

The mirror system 106 comprises two fold mirrors 118 and 120. Themirrors may be folded at an angle of about 11° to shorten the opticalpath length. The mirrors reflect light, as shown, of a wavelength, suchas about 450 to 650 nm, produced by the illumination source 110.

As shown in FIG. 2, the objective lens system 107 may comprise a pair of2-doublet achromats 121 and 122. In this embodiment, there are twodoublets for each imager of the image sensor system 108. The objectiveor the achromat 121, as shown in FIG. 1, includes doublet lenses 123 and124. The objective or the achromat 122 also includes two doublet lenses(not shown) configured in a similar fashion. Each 2-doublet objectivefunctions as an achromatic lens, thereby removing a substantial portionof chromatic aberration. That is, the objective lenses provide lateraland axial color correction for the wavelengths of interest, for example,about 450 to 650 nm. The lens mechanism 107 also provides a respectiveaperture stop 126 for each objective that defines an aperture light beamof imaging light rays forming an image of a fingerprint pattern. Othercolor-corrected lens configurations may be applied in the objective lensto accomplish focus of the image onto the image plane without coloraberration. For example, a six element objective comprising twodoublets, two singlets, and an aperture stop to provide higher CTF overa wider field of view may be employed.

The doublets may be mounted in a barrel arrangement 127. The barrelarrangement may have a diameter of about 1.50 inches and a length ofabout 3 inches.

The object plane field of view (FOV) is thus mapped to the image sensorsystem 108 through color corrected optics, including the fold mirrors.The optics configuration is telecentric at both the object plane and theimage plane to provide a broad region of high optical performance,exhibiting small blur spots throughout the FOV for all colors. Thevertical FOV, in various embodiments, may be between about 2 and 4inches.

In one embodiment, the image sensor system 108 comprises a pair of CMOSimage sensors 128 and 130. The image sensors are high-pixel densitycolor imagers. The image sensors employed may be three megapixel CMOSdevices manufactured by Micron Semiconductor. Alternatively, the imagesensors may use fewer than or more than three megapixels. Single imagersmay be employed depending on the horizonatal field of view to be imagedand the available imager horizontal and vertical pixels.

The image sensor or sensors provide digital output data at frame ratesof about 12 frames per second or greater. The frame rate is high enoughto avoid artifacts. The sensors are tilted to accomplish Scheimpflugcorrection of trapezoidal image distortion and variation in focus invertical FOV caused by the steep viewing angle. The sensor angle isbetween 2 and 11° and is a function of the object plane viewing angle120. The sensors may be tilted, in one embodiment, at an angle of about2.8° from the image axis. Each sensor views a portion of the total FOV.For instance, in the illustrated embodiment, each imager or sensor viewsapproximately half of the total FOV.

The imager sensor or sensors may use a RGB (red-green-blue) color matrixor a CMYG (cyan-magenta-yellow-green) color matrix. Also, color CCDimagers may be used in place of the color CMOS imagers.

As shown in FIG. 1, the illumination source 110 is arranged and operatedto illuminate the finger field 112. The illumination source 110 is amulti-color or non-monochromatic light source. The illumination source,in one embodiment, may be a white light source producing light in therange, for example, of about 450 to 650 nm. The illumination source, inother embodiments, may be a red and green; blue and green; cyan, magentaand green; cyan, green and yellow; or green, yellow, and magenta lightsource. The illumination source may be radiation sources such as lightemitting diodes (LEDs), cold cathode fluorescent tubes, or plasma lightpanels.

As shown in FIG. 3, the illumination source may be a light pipe 132. Thelight pipe includes light receiving surfaces 138 substantiallyorthogonal to a light input surface 134. The light output surface 136 isspaced from and substantially parallel to an outside light receiving orback surface 138. An open space or cavity 139 is formed between thelight output surface 136 and the back surface 138.

The light pipe may be constructed of clear acrylic. The outside surface138 of the light pipe, opposite the light output surface, may be opaquewhite. For example, this surface may be painted or coated to be opaque.Internal surfaces of the light pipe are constructed to provide uniformillumination of the painted surface.

A diffusing structure 140 is positioned at the output face 136 of thelight pipe. The diffusing structure diffuses minute irregularities inthe light output from the surface 136 and deflects undesirable lightentering the system through the object plane away from the light pipe.The diffusing structure may comprise an array of microprisms. Themicroprism array may be a Vikuiti Display Enhancement Film availablefrom the 3M Corporation, St. Paul, Minn. The microprism structure may bemolded into the front face 136 of the light pipe. Alternatively, themicroprism structure may be formed as a separate part form the surface136.

The light source, in one configuration, includes LED devices 140 mountedin respective reflecting end caps 142 mounted on or adjacent to thelight pipe light receiving surfaces 134. The devices 140 are high outputwhite (approximately 3000 K) LEDs. The LEDs produces white light in therange, for example, of about 450 to 650 nm.

The illumination source provides uniform illumination at the surface112. This reduces the amount of gain equalization necessary across theFOV, thereby increasing signal to noise and ensuring high grayscalecontrast. Additionally, the illumination scheme has the benefit ofweakly imaging latent fingerprint ridge residue left on the object plane112 by oil and other residue on the finger. Thus, this illuminationscheme provides a relatively high contrast ratio with low noise.

The CMOS imager outputs are digital with output data corresponding tored (R), green (G), and blue (B) components of the applied light. Thewhite light stimulates all RGB components with equal illumination, firstorder. Second order variations due to (1) illumination differences inthe three primary colors, (2) Bayer filter losses, and (3) imager RGBchannel gain imperfections are compensated for by first normalizing theRGB imbalances to accomplish equal outputs per channel across thedynamic range of the imagers followed by illumination equalization tocompensate for second order variations in the illumination profile.

As shown in FIG. 4, processing of image data, from imagers 108, derivedfrom imaging surface 112, is accomplished in a special purpose digitalsignal processing (DSP) computer 150. The computer 150 receives imageoutput data 152 from the fingerprint image sensors or imagers 128 and130. This processed image data conforms to the FBI specifiedrequirements, for example, the FBI Appendix F specifications. Theprocessed images are outputted to a host computer (not shown) via anIEEE 1394 Firewire link 154 for assembly into a record comprised of aset of images and textual data. An image grab 156 of the computer 150may be implemented using a technique such as that described in U.S. Pat.No. 5,748,766, entitled “Method and Device for Reducing Smear in RolledFingerprint Image,” or the technique described in U.S. Pat. No.4,933,976, entitled “System for Generating Rolled Fingerprint Images”,assigned to the assignee of the subject application, and which areherein incorporated by reference.

The computer 150 also controls, as represented by control box 158, theoperation of the image sensor system 108 and the illumination source110. The optics of the imaging system 100 are represented by box 160.

In another embodiment, data may be transferred to the host computerprior to processing into final fingerprint form. The final fingerprintprocessing would then be accomplished in software or a combination ofhardware and software on the host computer.

Also, an optional hand scanner 162 may be used in conjunction with theimaging system 100. The hand scanner operates under the control, controlbox 164, of the computer 150. The computer receives image output data166 from the hand scanner 162. The hand scanner 162 may be of the typedescribed in U.S. Pat. No. 6,175,407, entitled “Apparatus And Method ForOptically Imaging Features On The Surface Of A Hand”, assigned to theassignee of the subject application, and which is herein incorporated byreference.

A number of implementations and techniques have been described. However,it will be understood that various modifications may be made to thedescribed components and techniques. For example, advantageous resultsstill could be achieved if steps of the disclosed techniques wereperformed in a different order, or if components in the disclosedsystems were combined in a different manner, or replaced or supplementedby other components.

For example, the optical layout of the imaging system 100 may use onlyone fold mirror or more than two mold mirrors. It is also possible, inone embodiment, to eliminate the fold mirrors entirely by eitherreducing the front and back focal length or the objective lens and/orpackaging the optics system in a longer housing. The objective lenssystem may comprise some lens combination other than a pair of doublets.For instance, a combination of three pairs of doublets may be used toprovide higher CTF over a larger FOV. Also, objects other than a fingermay be imaged by the imaging device.

Additionally, instead of two imagers, only one imager may be employed. Asmaller prism could be used and the prism illuminated by an LED whitelight source. The FOV could be mapped to a single high-pixel densitycolor CMOS imager through color corrected optics with magnificationreduced to increase resolution. The single CMOS imager embodiment mayalso use a larger prism with magnification increased to yield a lowerresolution image but over a much larger FOV. In another configuration,the light source as discussed, may be multi-color providing more thenone wavelength of light through color-corrected optics to stimulate morethan one pixel color at the CMOS or CCD imaging device. For example, aCMYG (cyan-magenta-yellow-green) imager might be efficiently illuminatedwith a light source that would stimulate three of the four imagercolors, but not the fourth. The color correction in the optics would beeasier by virtue of the smaller spread of wavelengths that would have tobe supported and the illumination could be accomplished with bi-colorLEDs.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

1. A system for optically imaging an object comprising: an opticalplaten having an object receiving surface; a multi-color light sourcepositioned to illuminate the object receiving surface; a color imagingsystem having an image plane and positioned to receive light from theobject receiving surface to form an image of the object on the objectreceiving surface; and a lens mechanism to focus light from the objectreceiving surface onto the image plane and to provide color correctionof the focused light.
 2. The system of claim 1 wherein the lensmechanism includes an achromat.
 3. The system of claim 2 wherein thelens mechanism includes either one pair of doublets or one pair ofdoublets and a pair of singlets.
 4. The system of claim 1 wherein thelens mechanism removes a substantial portion of chromatic aberration. 5.The system of claim 1 wherein the lens mechanism includes an aperture.6. The system of claim 1 further including a folding mirror to directlight from the object receiving surface to the lens mechanism.
 7. Thesystem of claim 6 wherein there are two folding mirrors.
 8. The systemof claim 1 wherein the light provided by the light source has awavelength of between about 450 and 650 nanometers.
 9. The system ofclaim 1 wherein the imaging system further includes a pair of CMOSimagers.
 10. The system of claim 9 wherein the imagers are tilted at anangle from a normal.
 11. The system of claim 1 wherein the imagingsystem includes either a CCD or CMOS imager.
 12. The system of claim 1wherein 500 and 1,000 pixels per inch images are produced.
 13. Thesystem of claim 1 being configured to capture at least four-finger slap,single-finger slap, and rolled fingerprint images.
 14. The system ofclaim 1 wherein the light source is a white; red and green; blue andgreen; cyan, magenta and green; cyan, green and yellow; or green,yellow, and magenta light source.
 15. The system of claim 1 wherein thelight source is selected from a group consisting of a light emittingdiode, a cold cathode fluorescent tube, or a plasma panel illuminator.16. The system of claim 1 wherein the object is a finger.
 17. A systemfor optically imaging features on a surface of a hand comprising: anoptical plate means for forming a finger receiving surface; anon-monochromatic light source means for illuminating the fingerreceiving surface; a color imaging means for receiving light from thefinger receiving surface to form an image of a finger on the fingerreceiving surface; and a lens means for focusing light from the objectreceiving surface onto an image plane of the color imaging means and forproviding color correction of the focused light.
 18. A method of imagingan object comprising: receiving an object at an object receiving surfaceof an optical platen; illuminating the object receiving surface with amulti-color light source; collecting light from the object receivingsurface; and color correcting and focusing the collected light onto animage plane of a color imaging system to form an image of the object.19. The method of claim 18 wherein the received object is a finger. 20.The method of claim 18 wherein light from the light source illuminatingthe optical platen is incident on the optical platen at an angle withrespect to a normal to the object receiving surface which is less than aparticular critical angle.
 21. An illumination source comprising: alight output surface; a light receiving surface substantially orthogonalto the light output surface; a diffusing structure at the light outputsurface; and a non-monochromatic light source located adjacent to thelight output surface.
 22. The illumination source of claim 21 whereinthe diffusing structure is an array of microprisms.
 23. The illuminationsource of claim 21 wherein the light source produces white light; redand green light; blue and green light; cyan, magenta and green light;cyan, green and yellow light; or green, yellow, and magenta light. 24.The illumination course of claim 21 wherein a light source is positionedin respective reflecting end caps located at opposite sides of a cavityformed between the light output surface and a back surface.
 25. Theillumination source of claim 21 wherein the back surface is opaque andthe light output surface is clear.